The present invention relates generally to computer networking and more particularly to the internetworking requirements of Network layer protocols when switching or bridging frames between computer networks operating according to different Data Link layer protocols.
Computer communication models, such as the International Standards Organization (ISO) Open Systems Interconnection (OSI) model and the Institute of Electrical and Electronic Engineers (IEEE) model are known. The OSI model is a seven layer model that assumes an internetwork comprising a set of stations connected by physical media. A first, or "Physical" layer of the OSI model is concerned with the physical properties of the various media which can be used to directly connect stations. For example, media can be implemented on a point-to-point physical link connecting only two stations, or media can be implemented on a common link shared by many stations. A second, or "Data Link" layer of the model is concerned with how stations communicating on a physical link cooperate so as to efficiently and fairly use the bandwidth of that physical link. A third, or "Network" layer of the model is concerned with stations not directly connected by a common physical link. These "end-stations" communicate using intervening stations and intervening physical links which relay data between the endstations. The protocols at this Network layer are often referred to as "routing" protocols.
The OSI model does not require one-to-one correspondence between the communicating entities and the physical stations which make up the internetwork. These and other aspects are dealt with in the four remaining layers of the model: the Transport layer, the Session layer, the Presentation layer and the Application layer.
Ideally, in building an internetwork, an implementation of all tasks addressed by the various layers of the OSI model is provided. In practice, however, the size of the network (both geographically and number of stations) and the quality of service required (i.e., reliability, throughput, and so forth) can result in many internetworking issues being ignored or addressed by only marginal proprietary implementations. For example, if all of the end-stations in the internetwork can communicate on a common physical link which provides shared access to those stations, there is no need to implement a Network layer because the protocols of the Data Link layer are sufficient. This "shared access" model is implemented, for example, on a single Local Area Network (LAN) segment.
Thus, in practice, networks are built by taking LANs and WANs governed by different standards and combining them into internetworks using proprietary protocols and techniques. While these proprietary internetwork implementations can be described and analyzed according to the OSI model, they are not necessarily implementations of OSI protocol standards. Thus, proprietary protocol implementations may not be capable of interworking with each other. Still, so long as the proprietary techniques observe certain underlying standards common to LANs and WANs, they can coexist as distinct logical networks sharing a physical network infrastructure.
Some important LAN standards have been provided by the IEEE LAN 802 committees. The IEEE provides standards for LANs which divide the Physical and Data Link layers of the OSI model into two sub-layers. The sub-layers includes a media dependent Media Access Control (MAC) layer and a media independent Logical Link Control (LLC) layer. The MAC layer, or MAC protocol is media-specific and governs how directly connected stations access and share physical media. The LLC layer defines the format of the frames carried on the media. These IEEE 802 LAN committees have implemented standards for various physical media, such as CSMA/CD (IEEE 802.3) and Token ring (IEEE 802.5).
Other governing bodies have set forth LAN standards for other media. For example, the American National Standards Institute has implemented the ASNI X3T9.5 standards for communication utilizing Fiber Distributed Data Interface (FDDI) as the communication media.
Similarly, Wide Area Network (WAN) technologies are governed by standards bodies under the umbrella of the International Telecommunications Union (ITU). In many cases, the physical media underlying WANs are specified to use frame formats defined by ISO/IEEE for LANs.
The proliferation of different LAN and WAN standards and proprietary internetworking techniques has led to internetworking complications. One internetworking complication has arisen in the context of switching or bridging between disparate media. In forwarding frames between different media, a switch or bridge must often modify the frames to observe the IEEE standards for the underlying Physical and Data Link layer protocol of the media onto which a frame is being transferred. This modification involves changing the bit ordering of, i.e., "bitswapping", certain MAC fields contained in the frame. However, when these alterations of MAC fields are made independent of the Network layer address fields in the frame, the frame is susceptible to violating the expectations of the destination Network layer station regarding the bit ordering relationship between those fields, and the frame may not be recognized by the station.
More particularly, certain proprietary Network layer protocols dictate a particular relationship between the bit ordering of the MAC and Network address fields of a frame. However, encoding of addresses in the MAC header is dictated by LAN standards as well as these proprietary Network layer protocols. Thus, implementation of LAN standards by a switch or a bridge may lead to a frame being encoded in a way which violates the expectations of a proprietary Network layer standard. For instance, Novell's proprietary IPX Network layer protocol has a requirement for the bit ordering of addresses in the MAC and IPX headers which is not always observed by LAN standards when implemented in a mixed media environment. This has rendered switching and bridging of IPX frames between disparate media, such as between Token ring and FDDI or between Token ring and CSMA/CD, unworkable.
The need for a solution to the problem of violating expectations is particularly acute because of raised expectations of switch users with the introduction of technology such as Virtual LANs (VLANs). VLANs encourage a view where the physical topology and media is hidden in preference to the logical structure and use of the network by its end-users. Having been encouraged to view their networks in this way, users do not want to be informed that station X cannot communicate with station Y because station X is on a Token ring.
The problem can be generalized to all situations in which a frame may be subjected to inconsistent or overlapping standards and proprietary specifications imposed at the Data Link and Network layers of the OSI model. Accordingly, it would be desirable to find a solution that simply and efficiently recognizes frames that are susceptible to violating the expectations of a Network layer protocol and that applies an appropriate correction. It would be further desirable to provide such a solution without imposing undue latency in transferring frames.